If I interpreted what you are saying correctly, I have never been successful and trying to do this. In fact, the opposite is true. They will waitlist me on the transcon and not even bother with the domestic portion because – and I’ve been told this each time for the last few years – that if I cleared the domestic on an SWU, the eVIP-U will be “marked” as used.
so, I’ve taken my chances each time. I’d love to learn that you are correct for future upgrades. Have you seen this in writing anywhere? I’ve looked for it but I have not found it.

interesting, thanks. I’ve asked every time I called over several years and never any luck.

In my experience (and I should say that I haven’t used an SWU in the last year, so things there could have changed recently, but I have used mileage upgrades), you can still be waitlisted for the international leg and clear the domestic leg. The SWU will be encumbered—or “marked” as used—and you might have to call in to subsequently clear the other leg(s) if space becomes available (although that’s not unique to this circumstance); nonetheless, clearing the domestic leg does not mean you can’t use the same SWU to upgrade the international legs later.

I have never been able to pull an SWU, though, once a leg has cleared—even if the original fare class was available. Maybe that was something you could do back in the day (or with a high helix score), but I have not been able to do it.

find I fare availability on the lax-jfk transcon and you should be able to price it out at around 1950 usd.

sample. Change dates to something that also shows 1850 dollars.
https://goo.gl/flights/tLCft

sample via jfk outbound and via lax then dfw on inbound, ( to maximize miles earnings)
https://goo.gl/flights/S8Zjc